Perilous Journeys: the Science Behind Migration
AS WARM SUMMER EVENINGS give way to crisp fall nights, the skies over much of North America are filling with animals on the move—billions of songbirds, shorebirds, raptors, waterfowl, and insects winging their way south for the winter. In the oceans, whales, turtles, sharks, and other creatures are also voyaging toward winter homes, while land mammals from elk to bighorn sheep to caribou are tracing ancient routes to warmer altitudes and greener forage.
Among nature’s most astonishing phenomena, these seasonal mass movements of wildlife have fascinated scientists and others for millennia, yet researchers are only beginning to answer some of the most basic questions about them: What cues, internal and environmental, trigger animals to migrate? How do they navigate hundreds or thousands of miles, sometimes returning to the exact same beach, stream or patch of forest? Where do they stop along their routes, and what happens there?
Thanks in part to advancing technologies in the laboratory and the field, new findings are accumulating quickly. But a shadow hangs over the subject. Many migratory species, from songbirds to sharks, are in sharp decline. Some, such as right whales and sea turtles, are seriously endangered. The main causes are habitat destruction and the accompanying human-made obstacles that impede migrations, such as roads, dams, developments, fences, fishing gear, and tall structures. The newest threat, and potentially the worst, is climate change, which already seems to be disrupting the migratory cycle of some species. As Princeton University ecologist David Wilcove writes in a recent book, No Way Home: The Decline of the World’s Great Animal Migrations, “The irony is that just as the phenomenon of migration is slipping away, we are entering a golden age for studying it.”
MARTIN WIKELSKI, director of the Max Planck Institute for Ornithology in Germany and an associate professor of ecology at Princeton, calls the flood of new information about migration “mind-boggling,” but adds, “Aristotle speculated that migratory birds spent the winters in swamps, stuck in the mud. For many animals, we’re still about as dumb as Aristotle. We just don’t know.”
Wikelski is doing his bit to fix that. The pioneering scientist has collected the first data on individual migrating insects, for example. He and his team glued tiny radio tags onto 14 green darner dragonflies and followed them by car and airplane until the transmitters died. The insects surprised Wikelski by acting a lot like birds—or rather, birds act like the bugs, since insects are more than 100 million years older. Like birds, the dragonflies stopped to rest and refuel, took advantage of tail winds, and hunkered down when bad weather made travel inadvisable.
“It’s stunning,” says Wikelski. “It looks as if the general rules of migration could be similar between these different classes of animals on the wing.” It also looks as if the migratory urge is far more ancient than previously suspected.
Many creatures wander, but only some are true migrants. Most biologists define migration as repeated seasonal movement between breeding and nonbreeding grounds by the same individuals. The exceptions are certain insects, which complete the cycle in generational relays. Monarch butterflies, for instance, leave Mexico in the spring and stop in the southeastern United States, where they lay eggs and die. Their descendants undertake the next stage of the migration. By mid-summer the third or fourth generation has completed the trip to the northeastern United States and southern Canada, where they lay the eggs that will become the most amazing generation of all—the one that flies 3,000 miles in autumn to the same mountain forests in Mexico.
Until a few decades ago biologists did not know how monarchs and other migratory species find their way. A series of experiments in the mid-1970s—in which reversing magnetic fields around songbirds triggered them to fly in the wrong direction—revealed that the birds use Earth’s magnetic field to navigate. It is now known that some 50 species—including mammals, reptiles, amphibians, fish, and insects as well as birds—follow magnetic pathways, although scientists are still debating just how the animals detect magnetism.
Other navigational aids include circadian clocks, internal sun and star compasses, smells, and geographical features such as mountain ranges and coastlines. Most migrants seem to rely on a combination of these, depending on conditions. It turns out that monarchs, for example, navigate partly via an internal sun compass, but since the sun appears to move across the sky throughout the day, they must constantly correct their course. Steven Reppert, a professor of neurobiology at the University of Massachusetts School of Medicine, recently discovered how: The monarch’s sun compass communicates molecularly with its circadian clock. “There must be a genetic program that gets turned on in the migrating generation,” says Reppert.
THANKS TO TRACKING DEVICES, we now know the astounding distances that some migrants travel. Humpback whales shuttle more than 5,000 miles between breeding grounds off Central America and feeding grounds off Antarctica, the longest known migration by any mammal. An endangered leatherback turtle swam 12,740 miles across the Pacific, the current record for sea vertebrates. For sheer distance, nothing beats birds. Sooty shearwaters astonished scientists by flying more than 40,000 miles in a loop from New Zealand to Chile, Japan, Alaska, and California before a trans-Pacific trip back to New Zealand. The birds averaged more than 200 miles per day for 200 days. That pales compared to the bar-tailed godwit that flew 6,340 miles nonstop between New Zealand and North Korea, where it rested briefly before continuing to its breeding grounds in Alaska. At the other end of the scale, salamanders often scurry just a few hundred yards between their winter burrows and their breeding ponds.
Wikelski expects advances in satellite tracking technology to further transform research on migration. To that end, he recently helped start the ICARUS Initiative, whose twin goals are to launch a satellite devoted to migratory animals and to develop tiny, powerful transmitters that will allow researchers to follow creatures as small as insects across broad stretches of time and space. Until scientists can track individuals throughout their travels via satellite, many aspects of migration will stay hidden.
That frustrates Peter Marra, a research scientist at the Smithsonian Migratory Bird Center at the National Zoo in Washington, D.C. His strongest wish is to follow individual birds through the entire migratory cycle, including stopovers, to see how each phase influences the others. Meanwhile Marra gleans information indirectly from stable carbon and hydrogen isotopes. Because different habitats provide different diets—which leave distinctive isotopic “signatures” in feathers and tissue—he can use those signatures to detect how birds spend their summers and winters.
For 20 years Marra has been comparing two populations of American redstarts that winter in Jamaica, one in a lush mangrove forest, the other in dry scrub. The birds living the lush life eat well all winter, storing up reserves that enable them to migrate earlier in spring, giving them dibs on the best territories in North American breeding grounds. By contrast, the scrub birds eat less well and need more time to bulk up, which delays their departure and forces them into unclaimed territories typically containing poorer habitat. Because mangrove birds get a head start on breeding, they and their offspring may leave earlier in the fall and reclaim the best winter habitat in Jamaica—a pattern of haves and have-nots that may repeat itself, and affect breeding success, health and survival, through generations.
“Most researchers are focusing on just one period of the annual cycle—the breeding season,” says Marra. “But we’re seeing that the nonbreeding season drives a lot of what happens during rest of the cycle.” Its middle stages—the actual travel—remain terra incognita to scientists, whether for redstarts, leatherbacks, leafhoppers, or sharks. The sheer number of habitats used by different animals in different locales at different points in their migrations presents a formidable challenge to conserving those ecosystems.
YET CONSERVATION IS BADLY NEEDED. The habitats of migratory species nearly everywhere are under pressure from deforestation, farming, overfishing and expanding human populations. Human-made obstacles also hinder travel. Dams block salmon, ships hit whales, skyscrapers kill birds, and fences block animals such as deer, pronghorn, and bighorn sheep. Across the United States, the web of roads connecting city to suburb to exurb grows ever more dense, as does traffic, multiplying the hazardous barriers facing migratory animals.
John Bissonette, an ecologist for the U. S. Geological Survey and a professor at Utah State University, studies such obstacles and ways to mitigate them. “We’re not about to see a reduction in cars or speed,” he says, “so instead we deal with animal behavior.” In southern Utah, for example, traffic on two interstates was killing large numbers of migrating mule deer. In a pilot project funded by the U.S. Bureau of Land Management, Utah Department of Transportation and Utah Division of Wildlife Resources, Bissonette and student Silvia Rosa looked at the impact of installing underpasses and innovative right-of-way escape ramps along 25 miles of highway. Deer-vehicle collisions dropped by 95 percent. “We’ve essentially restored the migratory routes that come from the mountains in the east and cross the interstate to the west,” he says.
Most western migratory routes have already disappeared. In the Greater Yellowstone Ecosystem, for example, one pronghorn population has lost all but a single pathway between its summer and winter habitats. For more than 6,000 years, the animals have been making this seasonal journey between what is now Wyoming’s Upper Green River Basin and Grand Teton National Park. Though some 400 pronghorn still take the 300-mile round-trip, they must cross a gauntlet of highways, housing developments, drill pads, and more than 100 fences. If more obstacles block this last passage, especially at several choke points along the route, scientists say the continent’s longest known pronghorn migration (and the longest migration of any land mammal in the lower 48 states) will die.
For several years, conservationists have been campaigning to turn this pronghorn pathway into the world’s first protected migration corridor. Ninety percent of it is already on federal land. The idea is finally getting some traction. Earlier this year three federal agencies—the U.S. Fish and Wildlife, National Forest, and National Park services—signed a letter of general support, and in June, the Forest Service signed an amendment to a management plan pledging to protect pronghorn on the 45 miles of the corridor it manages. Meanwhile, the Western Governors’ Association has passed a resolution to protect wildlife corridors.
“It’s promising, and better than I would have thought possible five years ago,” says Joel Berger, a biologist for the Wildlife Conservation Society and University of Montana who has been involved in the corridor campaign. But he points out that the region’s pronghorn are facing a newer threat: gas field proliferation. “In just five years, there has been more than a tenfold increase in traffic in areas where pronghorn winter,” says Berger.
Migrations are also being challenged by climate change. Researchers are reporting new behaviors among migratory animals worldwide that may stem from higher temperatures—shifts in breeding ranges, mis-timing of cues and departures—with consequences that roll through the migratory cycle. “Since the wintering habitats are changing at different rates than more northerly habitats,” says Marra, “things get really out of sync.”
In the Netherlands, for example, some populations of pied flycatcher have crashed because the birds are arriving from African wintering grounds too late to feast on a once predictable bounty of caterpillars. Because of higher springtime temperatures in Europe, the insects are hatching earlier than they once did.
Changes in sea temperature, meanwhile, are expected to alter wind patterns and cause more frequent storms, stressing all flying migrants. Rising ocean temperatures also seem to be pushing organisms such as zooplankton and krill closer to the poles, with repercussions up the food chain, leaving whales and other animals either hungry or forced into longer migrations to find prey. And even a slight rise in sea level will submerge many atolls where marine turtles nest.
CAN MIGRATORY ANIMALS ADAPT? No one knows. Over the centuries, at least a few species have started, stopped, and restarted migrating, and some birds have recently shown flexibility by changing routes or timing or breeding sites in response to new environmental conditions. In the Midwest, for instance, ducks are arriving later in the fall and resting longer before continuing south, perhaps in response to higher temperatures. “But I think it would be dangerous to assume that all species can adapt,” says Wilcove, “or that they can respond to the full array of threats we are imposing on them.”
Wilcove points out that by 1890, the United States already had managed to destroy the greatest mammal and bird migrations ever known—60 million bison on the Great Plains and hundreds of millions of passenger pigeons east of the Mississippi. “History suggests that once a great migration is destroyed,” he says, “there is little prospect of recreating it.”
©Steve Kemper. All rights reserved. Cannot be reproduced without consent of the author.